State of charge optimizing device and assembled battery system including same

ABSTRACT

A state of charge optimizing device according to the present invention optimizes a state of charge of each of a plurality of cells which are connected in series to form an assembled battery, and conducts the optimization by discharging or charging a part or all of the plurality of cells so that the differences between the amount of charge after the optimization and the amount of charge in a predetermined state of charge become uniform.

The priority application Number 2007-251384, upon which this patentapplication is based, is hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to devices optimizing the states of chargeof a plurality of cells that form an assembled battery, and assembledbattery systems including the devices.

2. Description of Related Art

In recent years, an assembled battery has been widely used, for examplean assembled battery including a plurality of lithium-ion secondarycells connected in series is used in a hybrid vehicle as a power source.Discharge power of an assembled battery is limited by a cell with thelowest state of charge (SOC) among a plurality of cells that forms theassembled battery. Therefore, performance of an assembled batterydecreases due to the variation of SOC of the plurality of cells thatform the assembled battery.

Accordingly, a process to equalize the SOCs of the plurality of cells ofthe battery is required. In a conventional equalizing process, thevoltages across the respective cells that form an assembled battery(open-circuit voltages) are detected, and the lowest value or theaverage value of the detected cell voltages is used as a target valuefor the equalizing. Thus, electric discharge is conducted on the cellwhose voltage is above the target value for equalizing, therebyequalizing the SOCs of the plurality of cells that form the assembledbattery.

For example, as shown in FIG. 10 in which three cells 1 to 3 form anassembled battery, when the SOC of the cell 1 is the lowest beforeequalizing, the cells 2 and 3 are discharged to the SOC of the cell 1,thereby equalizing the SOC of the cells 1 to 3.

However, the inventor of the present invention has discovered in hisresearch that the full charging capacities of the plurality of cellsthat form the assembled battery vary due to the variationas-manufactured, or the variation attributed to temperature distributionwhen used, and therefore, the conventional equalizing process cannotsufficiently elicit the performance of the assembled battery.

For example, as shown in FIG. 11, in which three cells 1 to 3 havingdifferent full charging capacities form an assembled battery, in thecase where the cells 1 to 3 are equalized when the SOC of the cells isapproximately 70 percent, since the discharge amounts per unit time ofthe cells 1 to 3 are uniform, when the SOC of the cell 1 reaches 50percent due to discharge of the assembled battery after theequalization, the SOC of the cells 1 to 3 varies.

Also, in the case where the cells 1 to 3 are equalized when the SOC ofthe cells is approximately 30 percent, since the charge amounts per unittime of the cells 1 to 3 are uniform, when the SOC of the cell 1 reaches50 percent due to charge of the assembled battery after theequalization, the SOCs of the cells 1 to 3 vary.

The discharge characteristic of the battery decreases as the SOCdecreases, and the charge characteristic decreases as the SOC increasesas shown in FIG. 12. Therefore, the higher performance of the assembledbattery can be obtained when the SOCs of the plurality of cells areequalized with the SOC in which both the discharge characteristic andcharge characteristic are favourable and well balanced, such asapproximately 50 percent, rather than with a high or low SOC.

With the conventional equalizing process, in the case where the SOCs ofa plurality of cells are equalized when the SOC is approximately 70percent as described above, for example, since the SOCs of the cellsvary when the SOC of the cells decreases to around 50 percent due todischarge of the assembled battery after the equalization, theperformance of the assembled battery cannot be sufficiently elicited.

Accordingly, the equalization with the SOC of the plurality of cells ofaround 50 percent is considered. It is desirable to equalize theassembled battery of a hybrid car or the like when the car is stoppedand the battery is not charged or discharged in order to accuratelymeasure the amount of charge. However, the hybrid car or the like is notnecessarily stopped when the SOC of the assembled battery is around 50percent. It is therefore problematic to do the equalization when the caris stopped in that the equalization is done very infrequently.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a state of chargeoptimizing device capable of sufficiently eliciting the performance ofan assembled battery that includes a plurality of cells with differentfull charging capacities and an assembled battery system including thedevice.

A state of charge optimizing device according to the present inventionis a device optimizing the state of charge of each of a plurality ofcells connected in series to form an assembled battery, and the devicesets an equalization target value in view of the full charging capacityfor each of a part or all of the cells to equalize the battery bydischarging or charging each of the cells in accordance with the setequalization target value.

In particular, the equalization target value is set to the amount ofcharge, or a value corresponding thereto, such that the differencebetween the amount of charge after optimization and the amount of chargein a predetermined state of charge is uniform among the plurality ofcells.

According to the state of charge optimizing device of the presentinvention, after optimization, the difference between the amount ofcharge at the time and the amount of charge in the predetermined stateof charge is uniform among the plurality of cells that form an assembledbattery. Therefore, when the assembled battery is discharged or chargedthereafter, the states of charge of the plurality of cells will beuniform in the predetermined state of charge. Here, the predeterminedstate of charge is set to a state of charge in which both the dischargecharacteristic and charge characteristic of the assembled battery arepreferable and well balanced, such as 40 to 60 percent to sufficientlyelicit the performance of the assembled battery.

In particular, the state of charge optimizing device of the presentinvention comprises:

a charging and discharging unit capable of charging and/or dischargingeach of the cells;

an amount of charge difference calculating unit calculating thedifference between the current amount of charge and the amount of chargein the predetermined state of charge for at least one of the pluralityof cells;

an optimization target value setting unit setting an optimization targetvalue in accordance with the amount of charge difference calculated bythe amount of charge difference calculating unit for each of a part orall of the plurality of cells; and

an optimizing processing unit making the charge and discharge unitconduct charging or discharging in accordance with the set optimizationtarget value for each of a part or all of the plurality of cells whenthe optimization target value is set.

According to the particular configuration described above, theoptimization target value set for each of the cells is an amount ofcharge, or a value corresponding thereto, close to the amount of chargeat the time. Therefore, the charge or discharge amount can be small.

Also in particular, the optimization target value setting unit sets theoptimization target value for each of the cells other than a referencecell which is the at least one cell of the plurality of cells, andcomprises:

a first processing unit adding the amount of charge difference of thereference cell calculated by the amount of charge difference calculatingunit to the amount of charge in the predetermined state of charge foreach of the cells other than the reference cell; and

a second processing unit setting the optimization target value to theamount of charge calculated by the first processing unit or a valuecorresponding thereto for each of the cells other than the referencecell.

In the particular configuration described above, the optimization targetvalue is set to the amount of charge, or a value corresponding thereto,obtained by adding the difference between the amount of charge of thereference cell at the time and the amount of charge in the predeterminedstate of charge to the amount of charge in the predetermined state ofcharge for each of the cells other than the reference cell of theplurality of cells that form the assembled battery. Each of the cellsother than the reference cell is discharged or charged in accordancewith the optimization target value. As a result, the difference betweenthe amount of charge after optimization and the amount of charge in thepredetermined state of charge of each of the cells other than thereference cell is the same as the difference between the amount ofcharge and the amount of charge in the predetermined state of thereference cell. Therefore, when the assembled battery is discharged orcharged thereafter, the states of charge of the plurality of cells areuniform in the predetermined state of charge.

In particular, the value corresponding to the amount of charge is astate of charge or a cell voltage. The second processing unit of theoptimization target value setting unit converts the amount of chargecalculated by the first processing unit into a state of charge or a cellvoltage, and then sets the state of charge or the cell voltage obtainedfrom the conversion as the optimization target value.

In the particular configuration described above, the target amount ofcharge calculated by the first processing unit is converted into thestate of charge or the cell voltage. Here, the state of charge can becalculated by dividing the amount of charge by the full chargingcapacity and then multiplying the result by the value of 100. Also, thecell voltage and the state of charge have a certain relationship, andtherefore when the relationship between the cell voltage and the stateof charge is preliminary defined, the cell voltage can be derived fromthe state of charge in accordance with the relationship. After that, thestate of charge or the cell voltage obtained by the conversion describedabove is set as the target optimization value.

More particularly, the amount of charge difference calculating unitcalculates the amount of charge difference by subtracting the amount ofcharge in the predetermined state of charge from the current amount ofcharge for each of the cells that form the assembled battery. Theoptimization target value setting unit specifies a cell with thesmallest amount of charge difference calculated by the amount of chargedifference calculating unit as the reference cell from the plurality ofcells that form the assembled battery.

According to the particular configuration described above, theoptimization target value set for each of the cells other than thereference cell is an amount of charge, or a value corresponding thereto,which is below the amount of charge at the time. Therefore theoptimization can be conducted only by discharging. Thus, only adischarging means is required and the structure is simple compared to astate of charge optimizing device equipped with both discharging meansand charging means.

Further particularly, the amount of charge difference calculating unitcalculates the difference between the current amount of charge and theamount of charge in the predetermined state of charge for each of thecells that form the assembled battery, and the state of chargeoptimizing device comprises:

an identifying unit identifying a maximum value and a minimum value fromthe amount of charge difference calculated by the amount of chargedifference calculating unit;

a calculating unit calculating the difference between the maximum valueand the minimum value identified by the identifying unit; and

a determination unit determining whether or not a value calculated bythe calculating unit is above a predetermined threshold,

the optimization target value setting unit setting the optimizationtarget value when the calculated value is determined to be above thepredetermined threshold by the determination unit.

In the particular configuration described above, when the variationrange of the difference between the current amount of charge and theamount of charge in the predetermined state of charge among theplurality of cells that form the assembled battery exceeds thepredetermined threshold, the optimization target value is set and theoptimizing process is conducted.

An assembled battery system according to the present invention comprisesan assembled battery that includes a plurality of cells connected inseries, and a state of charge optimizing device for optimizing the stateof charge of each of the cells forming the assembled battery, and adoptsthe state of charge optimizing device of the present invention describedabove as the state of charge optimizing device.

As described, according to the state of charge optimizing device of thepresent invention and the assembled battery system including the stateof charge optimizing device, it is possible to sufficiently elicit theperformance of the assembled battery that includes the plurality ofcells with different full charging capacities.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating a configuration of a batterysystem according to the present invention;

FIG. 2 is a graph which explains an optimizing process of the presentinvention;

FIG. 3 is a graph which explains the effect of the optimizing process ofthe present invention;

FIG. 4 is a flowchart showing the procedure of the optimizing process ofthe present invention;

FIG. 5 is a flowchart showing the concrete procedure of a determiningprocess of necessity of the optimizing by a state of charge optimizingdevice of the first embodiment;

FIG. 6 is a flowchart showing the concrete procedure of an optimizationtarget voltage value calculating process by the state of chargeoptimizing device;

FIG. 7 is a graph which explains timing of conducting an optimizingprocess in a state of charge optimizing device of the second embodiment;

FIG. 8 is a flowchart showing the concrete procedure of a determiningprocess of necessity of the optimizing by the state of charge optimizingdevice;

FIG. 9 is a flowchart showing the concrete procedure of an optimizationtarget voltage value calculating process by the state of chargeoptimizing device;

FIG. 10 is a graph which explains a conventional equalizing process;

FIG. 11 is a graph which explains a problem of the conventionalequalizing process; and

FIG. 12 is a graph showing a charge and discharge characteristic of anassembled battery.

DETAILED DESCRIPTION OF THE INVENTION

The present invention implemented in a battery system of a hybrid car isdescribed below on the basis of two embodiments.

First Embodiment

As shown in FIG. 1, the battery system according to the presentinvention comprises an assembled battery that includes a plurality ofcells 1 which are lithium-ion secondary cells and connected in series(three cells in the example of the drawing), and a state of chargeoptimizing device 2 for optimizing the state of charge of the assembledbattery. The electrical power can be supplied to a load 3 from theassembled battery.

A discharge circuit 21 which includes a resistor R and a switch SWconnected to each other in series and a voltage measuring circuit 22which measures the voltage across each cell (open-circuit voltage) areconnected to both sides of each of the cells 1.

Values measured by each of the voltage measuring circuits are suppliedto a control circuit 20 and the control circuit 20 calculatesoptimization target voltage values as described below based on themeasured values, and then controls discharging of each of the dischargecircuits 21 based on the calculated optimization target voltage valuesand the values measured by the respective voltage measuring circuits 22.

In the optimizing process by the state of charge optimizing device 2, areference cell is a cell with the smallest amount of charge differenceamong the plurality of cells that form the assembled battery. The amountof charge difference can be obtained by subtracting the amount of chargein a predetermined SOC from the amount of charge at the time. A targetamount of charge is set for each of the cells other than the referencecell by adding the amount of charge difference of the reference cell tothe amount of charge in the predetermined SOC. And then the optimizationis conducted by discharging each of the cells other than the referencecell by the discharge circuit 21. Here, the predetermined SOC is set sothat both the discharge characteristic and charge characteristic arefavorable and well balanced. It is set to 50 percent in this embodiment.

For example, as shown in FIG. 2, with the assembled battery thatincludes three cells 1 to 3 having different full charging capacities,among the amount of charge differences of the three cells 1 to 3, ΔAh1,ΔAh2, and ΔAh3, when the cell 3 has the smallest amount of chargedifference ΔAh3, the cell 3 is the reference cell. The target amounts ofcharge for the cell 1 and cell 2 are set to values DAh1 and DAh2respectively, which are obtained by adding the amount of chargedifference ΔAh3 to the amounts of charge in the SOC of percent. Andthen, the optimization is conducted by discharging the cell 1 and cell 2to the respective target amounts of charge. As a result, among the cells1 to 3, the difference between the amount of charge after the optimizingprocess and the amount of charge when the SOC is 50 percent is uniform,whereby discharging of the assembled battery thereafter makes the SOCsof cells 1 to 3 uniform at 50 percent. When the assembled battery isfurther discharged or charged thereafter, the SOC of the cell 3, whichhas the smallest full charging capacity among the cells 1 to 3, willreach 0 percent or 100 percent the earliest.

FIG. 4 shows the procedure of the optimizing process conducted by thecontrol circuit 20 of the state of charge optimizing device 2 when theignition switch of a hybrid car is set to OFF. When the ignition switchis set to OFF first, in step S1, the control circuit 20 measures theopen-circuit voltage of each of the cells that form the assembledbattery after a predetermined period such as one hour. And then in stepS2, it determines whether or not it is necessary to conduct theoptimizing process. The particular procedure of the determining processin step S2 is to be described later. Subsequently in step S3, thecontrol circuit 20 determines whether or not the optimizing process isdetermined to be necessary in step S2 and when it determines NO, theprocess returns to step S1.

In contrast, when it is determined that conducting the optimizingprocess is necessary and therefore determined YES in step S3, thecontrol circuit 20 calculates an optimization target voltage in step S4,and then in step S5, starts discharging a cell with a voltage above theoptimization target voltage calculated in step S4. The particularprocedure of the optimization target voltage calculation process in stepS4 is to be described later.

Subsequently in step S6, after a predetermined period, such as 30minutes, the control circuit 20 measures the open-circuit voltage ofeach of the cells that form the assembled battery, and then in step S7,it terminates discharging the cell whose open-circuit voltage reachesthe optimization target voltage calculated in step S4. Next, in step S8,it determines whether or not it has terminated discharging all the cellswith a voltage above the optimization target voltage. When it determinesNO, the process returns to step S6. When it terminates discharging allthe cells thereafter, it determines YES in step S8 and terminates theprocedure.

FIG. 5 shows the concrete procedure of the determining process ofnecessity of optimization in step S2 described above. In the descriptionbelow, the predetermined SOC is indicated as N percent and referred toas a reference SOC.

In the determining process, after initializing a cell number i to 0 instep S11, the control circuit 20 estimates the current SOC[i] of thecell with the cell number i from the open-circuit voltage thereof. Here,the SOC can be estimated from the open-circuit voltage by referring to atable stored in a memory, which shows the relationship between theopen-circuit voltage and SOC.

Next in step S13, for the cell with the cell number i, the amount ofcharge difference ΔAh[i] between the current amount of charge and theamount of charge in the reference SOC (=N percent) is calculated fromthe full charging capacity Q[i] and the SOC[i] estimated in step S12 byusing the formula 1 below.

ΔAh[i]=Q[i]×(SOC[i]−N)  Formula 1

Subsequently in step S14, the cell number is increased by one, and thenin step S15, the control circuit 20 determines whether or not the cellnumber i conforms to the number of cells n that form the assembledbattery. When it determines NO, the process returns to step S12 and theprocedure described above is repeated.

Thereafter, when the amount of charge difference ΔAh[i] between thecurrent amount of charge and the amount of charge in the reference SOC(=N percent) is calculated for all the cells that form the assembledbattery, the control circuit 20 determines YES in step S15, and then instep S16, identifies the minimum value min ΔAh and the maximum value maxΔAh from among the amount of charge differences ΔAh[i] calculated forall the cells that form the assembled battery.

Subsequently in step S17, the control circuit 20 determines whether ornot the difference between the minimum value min ΔAh and the maximumvalue max ΔAh is above a predetermined threshold value X. When itdetermines NO, it determines that it is not necessary to conduct theoptimizing process in step S18, and terminates the procedure describedabove, while when it determines YES, it determines that it is necessaryto conduct the optimizing process in step S19 and terminates theprocedure described above.

According to the procedure described above, when the SOCs of theplurality of cells that form the assembled battery vary a great deal andthe difference between the minimum value min ΔAh and the maximum valuemax ΔAh calculated for the plurality of cells is above a predeterminedthreshold value X, it is determined that the optimizing process isnecessary.

FIG. 6 shows the concrete procedure of the optimization target voltagevalue calculating process in step S4 of FIG. 4. In this calculatingprocess, first in step S21, the control circuit 20 specifies a cell fromamong the plurality of cells that form the assembled battery as areference cell, and sets the cell number of the reference cell to thereference cell number K. Here, the cell with the smallest amount ofcharge difference ΔAh[i] calculated in step S13 of FIG. 5 is specifiedas the reference cell.

Next, after initializing the cell number i to 0 in step S22 of FIG. 6,the control circuit 20 determines whether or not the cell number iconforms to the reference cell number K in step S23. When it determinesNO, the process proceeds to step S24 and the control circuit 20calculates the optimization target amount of charge DΔAh[i] for the cellwith the cell number i from the full charging capacity Q[i], thereference SOC (=N percent) and the amount of charge difference ΔAh[K] ofthe reference cell by using the formula 2 below. Here, (Q[i]×N÷100) inthe formula 2 below is the amount of charge in the reference SOC.

ΔDAh[i]=Q[i]×N÷100+ΔAh[K]  Formula 2

Subsequently in step S25, the control circuit 20 converts theoptimization target amount of charge ΔDAh[i] calculated for the cellwith the cell number i in step S24 into the SOC by using the formula 3below.

DSOC[i]=ΔDAh[i]÷Q[i]  Formula 3

Next in step S26, the control circuit 20 converts the optimizationtarget SOC calculated for the cell with the cell number i in step S25into a voltage value, and then the process proceeds to step S27. Here,the SOC can be converted into the voltage value by referring to a tablestored in a memory, which shows the relationship between the SOC and thevoltage value.

In addition, when the cell number i conforms to the cell number K andtherefore it is determined YES in step S23, the process proceeds to stepS27, bypassing steps 24 to 26.

After the cell number i is increased by one in step S27, the controlcircuit 20 determines whether or not the cell number i conforms to thenumber of cells n that form the assembled battery in step S28. When itdetermines NO, the process returns to step S23 and the proceduredescribed above is repeated.

When the optimization target voltage value is calculated for all of theplurality of cells that form the assembled battery other than thereference cell, the control circuit 20 determines YES in step S28 andterminates the procedure.

According to the state of charge optimizing device 2 of the presentinvention, when the assembled battery is discharged or charged after theoptimizing process as described above, the SOCs of the plurality ofcells that form the assembled battery is uniform at 50 percent, whereboth the discharge characteristic and charge characteristic arefavorable and well balanced. When the assembled battery is furtherdischarged or charged thereafter, the cell with the smallest fullcharging capacity will reach 0 percent or 100 percent the earliest amongthe plurality of cells, thereby obtaining the maximum charging anddischarging capacity of the assembled battery. Therefore, it is possibleto maximally elicit the performance of the assembled battery thatincludes the plurality of cells having different full chargingcapacities.

Also, according to the state of charge optimizing device of the presentinvention, it is possible to conduct the optimizing process regardlessof the SOCs of the plurality of cells that form the assembled battery.

Further, among the plurality of cells that form the assembled battery,the cell with the smallest amount of charge difference, which isobtained by subtracting the amount of charge when the SOC is 50 percentfrom the amount of charge at the time, is specified as the referencecell. Therefore, the optimization target amount of charge calculated foreach of the cells other than the reference cell is below the amount ofcharge at the time, whereby the optimizing process can be conducted byonly discharging. Accordingly, only the discharge circuit 21 isrequired, and the configuration is simple compared to a state of chargeoptimizing device with both the discharge circuit and charge circuit.

Second Embodiment

While the state of charge optimizing device of the first embodiment isto conduct the optimizing process when the SOCs of the plurality ofcells that form the assembled battery vary a great deal, the state ofcharge optimizing device of this embodiment is to conduct the optimizingprocess when the amount of charge of a cell of the plurality of cellsthat form the assembled battery falls below or exceeds a lower limit andan upper limit which are determined in accordance with the amount ofcharge of the cell with the smallest full charging capacity.

The configuration of the state of charge optimizing device in thisembodiment is the same as that in the first embodiment except for thecontrol circuit. Therefore, the description thereof is omitted. Also,the overall procedure of the optimizing process conducted by the controlcircuit of this embodiment is the same as the procedure described in thefirst embodiment shown in FIG. 4. Therefore, the description thereof isomitted.

The charging and discharging capacity of the assembled battery isdetermined by the cell with the smallest full charging capacity amongthe plurality of cells that form the assembled battery. In other words,in the case of charging the assembled battery, the maximum chargingcapacity is obtained when the SOC of the cell with the smallest fullcharging capacity reaches 100 percent the earliest, and in the case ofdischarging the assembled battery, the maximum discharging capacity isobtained when the SOC of the cell with the smallest full chargingcapacity reaches 0 percent the earliest.

In order for the SOC of the cell with the smallest full chargingcapacity to reach 0 percent or 100 percent the earliest, the amount ofcharge of the cells other than this cell should be within a range with alower limit which is the amount of charge of the cell with the smallestfull charging capacity and an upper limit which is the amount of chargeobtained by adding the difference of the full charging capacitiesbetween the cell with the smallest full charging capacity and each ofthe cells other than this cell to the amount of charge of the cell withthe smallest full charging capacity.

For example, in the case where the assembled battery includes threecells 1 to 3 as shown in FIG. 7, the amount of charge of the cell 1should be within the range which is the amount of charge of the cell 3Ah3 or above, and the amount of charge Aho1 or below, which is obtainedby adding the difference a1 between the full charging capacities of thecell 1 and cell 3 to the amount of charge of the cell 3 Ah3. The amountof charge of the cell 2 should be within the range which is the amountof charge of the cell 3 Ah3 or above, and the amount of charge Aho2 orbelow, which is obtained by adding the difference a2 between the fullcharging capacities of the cell 2 and cell 3 to the amount of charge ofthe cell 3 Ah3.

Accordingly, in the state of charge optimizing device of thisembodiment, the optimizing process is conducted when the amount ofcharge of any of the plurality of cells that form the assembled batterydeviates the range described above.

FIG. 8 shows the concrete procedure of a determining process ofnecessity of optimization conducted by the control circuit of thisembodiment. In this determining process, first in step S31, the cellnumber i is initialized to zero, and then the current SOC[i] of the cellis estimated from the open-circuit voltage of the cell with the cellnumber i in step S32. Here, the SOC can be estimated by referring to atable stored in a memory, which shows the relationship between theopen-circuit voltage and the SOC.

Next in step S33, for the cell with the cell number i, the currentamount of charge Ah[i] is calculated from the full charging capacityQ[i] and the SOC[i] estimated in step S32, by using the formula 4 below.

Ah[i]=Q[i]×SOC[i]/100  Formula 4

Subsequently in step S34, the cell number i is increased by one, and thecontrol circuit determines whether or not the cell number i conforms tothe number of cells n that form the assembled battery in step S35. Whenit determines NO, the process returns to step S32 and the proceduredescribed above is repeated.

After that, when the current amounts of charge Ah[i] of all the cellsthat form the assembled battery are calculated, it determines YES instep S35, and next in step S36, identifies the minimum value minq fromamong the calculated current amounts of charge of all the cells thatform the assembled battery.

Subsequently in step S37, it determines whether or not the minimum valueminq identified in step S36 is below the current amount of charge of thecell with the smallest full charging capacity, Ah[J] (J is the cellnumber of the cell with the smallest full charging capacity). When itdetermines YES, it determines that the optimizing process is necessaryin step S38 and terminates the procedure.

When the minimum value minq is the current amount of charge of the cellwith the smallest full charging capacity Ah[J] or above and therefore itdetermines NO in step S37, it initializes the cell number i to zero instep S39, and then in step S40, it determines whether or not the currentamount of charge of the cell with the cell number i Ah[i] is above theupper limit which is obtained by adding the difference between the fullcharging capacities of the cell with the cell number i and the cell withthe smallest full charging capacity (Q[i]-Q[J]) to the current amount ofcharge of the cell with the smallest full charging capacity Ah[J]. Whenit determines NO, the cell number i is increased by one in step S42, andthen it determines whether or not the cell number i conforms to thenumber of cells n in step S43. When it determines NO, the processreturns to step S40 and the procedure described above is repeated.

During repeating the procedure described above, when it determines YESin step S40, it determines that the optimizing process is necessary instep S41 and terminates the procedure.

In contrast, when it determines NO in step S40 for all the cells thatform the assembled battery, finally it determines YES in step S43 andthen in step S44, it determines that the optimizing process is notnecessary and terminates the procedure.

In accordance with the procedure described above, in the case where theamount of charge of any of the plurality of cells that form theassembled battery is below the lower limit, or above the upper limit,the control circuit determines that optimizing process is necessary.

In contrast, in the case where the amount of charge of none of theplurality of cells that form the assembled battery is below the lowerlimit, or above the upper limit, the control circuit determines thatoptimizing process is not necessary.

FIG. 9 shows the concrete procedure of an optimization target voltagevalue calculating process conducted by the control circuit of thisembodiment. In this calculating process, after initializing the cellnumber i to zero in step S51, the control circuit, in step S52,calculates the amount of charge difference ΔAh[i] between the currentamount of charge and the amount of charge in the reference SOC (=Npercent) from the full charging capacity Q[i] and the SOC[i] estimatedin step S32 of FIG. 8, by using the formula 1 stated above.

Subsequently, after increasing the cell number i by one in step S53, thecontrol circuit determines whether or not the cell number i conforms tothe number of cells n that form the assembled battery in step S54. Whenit determines NO, the process returns to step S52 and the proceduredescribed above is repeated.

Thereafter, when the amount of charge difference ΔAh[i] between thecurrent amount of charge and the amount of charge in the reference SOC(=N percent) is calculated for all the cells that form the assembledbattery, it determines YES in step S54 and the process proceeds to stepS55.

And then, by conducting the procedure from steps S55 to S62, theoptimization target voltage value is calculated for all the cells thatform the assembled battery other than the reference cell. Here, theprocedure from steps S55 to S62 is the same as that from steps S21 toS28 shown in FIG. 6 and conducted by the control circuit of the firstembodiment. Therefore the description thereof is omitted.

In accordance with the state of charge optimizing device of thisembodiment, since the optimizing process is conducted with the timingdescribed above, it is possible to prevent the SOCs of the cells otherthan the cell with the smallest full charging capacity from reaching 0percent or 100 percent the earliest. Also, as in the first embodiment,when the assembled battery is discharged or charged after the optimizingprocess, all the SOCs of the plurality of cells become 50 percent. Whenthe assembled battery is further discharged or charged thereafter, theSOC of the cell with the smallest full charging capacity reaches 0percent or 100 percent the earliest, thereby eliciting the performanceof the assembled battery maximally.

In addition, in the embodiments above, the optimizing process isconducted based on the optimization target voltage value converted fromthe optimization target amount of charge. However, it is also possibleto adopt the configuration in which the optimizing process is conductedbased on the optimization target amount of charge, or the configurationin which the optimizing process is conducted based on the optimizationtarget SOC.

Also, in the embodiments above, the optimization is conducted bydischarging each cell until the cell voltage becomes equivalent to theoptimization target voltage value in a state where the ignition switchof a hybrid car is set to OFF. However, it is also possible to adopt aconfiguration in which the optimization is conducted during charging anddischarging the assembled battery. In such a configuration, anintegration value of the charging or discharging amount since the targetvalue is set is referred to as P (when it is charging, P>0, and when itis discharging, P<0), and the optimization target amount of charge DAhis kept modified using the formula (DAh+P). Accordingly discharging orcharging is conducted until the amount of charge, or the valuecorresponding thereto, of each cell becomes the optimization targetamount of charge or the value corresponding thereto.

Further, in the embodiments above, the optimization is conducted by onlydischarging the assembled battery, specifying a cell with the smallestthe value as the reference cell. Here, the value is obtained bysubtracting the amount of charge when the SOC is 50 percent from theamount of charge at the time. However, it is also possible to adopt theconfiguration in which the optimization is conducted by only chargingthe assembled battery, specifying a cell with the greatest value as thereference cell, or the configuration in which optimization is conductedby discharging and charging the assembled battery, specifying a cellother than the cells with the smallest and greatest values as thereference cell. The values here are obtained in the same manner asdescribed above.

In the embodiments above, for each of the cells other than the referencecell, the optimization target amount of charge is calculated by addingthe difference between the current amount of charge and the amount ofcharge in the predetermined SOC to the amount of charge in thepredetermined SOC. However, it is possible to obtain the optimizationtarget amount of charge by calculating the difference between thecurrent amount of charge and the amount of charge in the predeterminedSOC for all the cells that form the assembled battery, and thencalculating the average value of the calculated differences to be addedto the amount of charge in the predetermined SOC for each of the cells.

Also, in the embodiments above, the predetermined SOC is set to 50percent. However, it is not limited to 50 percent, and it is possible toset the SOC to any value which can sufficiently elicit the performanceof the assembled battery depending on the configuration orcharacteristic of the assembled battery. For example, with the state ofcharge optimizing device of the assembled battery in which dischargingshould be stopped at X1 percent (X1>0) and charging should be stopped atX2 percent (X2<100), the predetermined SOC (=N percent) is set to thevalue obtained by calculating with the formula 5 below.

N=(X1+X2)/2  Formula 5

Still further, in the embodiments above, each cell is dischargedseparately. However, it is also possible to adopt a configuration inwhich the optimization target value is set for a plurality of cellswhich are one module, and charging and/or discharging is conducted forevery module to conduct the optimization. In a state of chargeoptimizing device for an assembled battery that includes many cells,adopting such a configuration can realize a small circuit size.

Furthermore, the state of charge optimizing device of the presentinvention can be used for not only the assembled battery comprisinglithium ion secondary cells, but also other kinds of assembledbatteries.

1. A state of charge optimizing device for optimizing a state of chargeof each cell of a plurality of cells which are connected in series toform an assembled battery, wherein the device sets an optimizationtarget value in accordance with a full charging capacity for each cellof a part or all of the plurality of cells, and conducts optimization byconducting discharging or charging in accordance with the setoptimization target value.
 2. The state of charge optimizing deviceaccording to claim 1, wherein the optimization target value is an amountof charge, or a value corresponding thereto, such that the differencebetween the amount of charge after optimization and the amount of chargein a predetermined state of charge is uniform among the plurality ofcells wherein the predetermined state of charge is set so that both adischarge characteristic and a charge characteristic of the assembledbattery are favorable and well balanced.
 3. The of charge optimizingdevice according to claim 1, wherein the optimization target value is anamount of charge, or a value corresponding thereto, such that thedifference between the amount of charge after optimization and theamount of charge in a predetermined state of charge is uniform among theplurality of cells, comprising: a charging and discharging unit capableof charging and/or discharging each cell; an amount of charge differencecalculating unit calculating the difference between the current amountof charge and the amount of charge in the predetermined state of chargefor at least one of the plurality of cells; an optimization target valuesetting unit setting an optimization target value in accordance with theamount of charge difference calculated by the amount of chargedifference calculating unit for each cell of a part or all of theplurality of cells; and an optimizing processing unit making thecharging and discharging unit conduct charging or discharging inaccordance with the set optimization target value for each cell of apart or all of the plurality of cells when the optimization target valueis set.
 4. The of charge optimizing device according to claim 3, whereinthe amount of charge difference calculating unit calculates thedifference between the current amount of charge and the amount of chargein the predetermined state of charge for each of the cells that form theassembled battery, the state of charge optimizing device comprising: anidentifying unit identifying a maximum value and a minimum value fromthe amount of charge difference calculated by the amount of chargedifference calculating unit; a calculating unit calculating thedifference between the maximum value and the minimum value identified bythe identifying unit; and a determination unit determining whether ornot a value calculated by the calculating unit is above a predeterminedthreshold, the optimization target value setting unit setting theoptimization target value when the calculated value is determined to beabove the predetermined threshold by the determination unit.
 5. Anassembled battery system comprising an assembled battery that includes aplurality of cells connected in series, and the state of chargeoptimizing device according to claim
 1. 6-9. (canceled)